Microsoft Provides Peek at Spyware Detection Data

In the first three months after shipping the Windows Defender Beta 2 anti-spyware application, Microsoft detected 22 million pieces of adware/spyware programs, resulting in roughly 14 million removals.

Personally, I never noticed any removal done by Windows Defender, when I used it under its previous name (MSAS).
Windows Defender was more a still life icon on my computer.
14 million removals proves at least that it does something.